(a) Field of the Invention
This invention broadly relates to novel seismograms or sections for use in seismic prospecting. More particularly, it relates to variable-area seismograms and to specific combinations thereof to produce a single seismic section--especially for the purpose of correlating, interpreting, or otherwise translating data on the seismograms to more easily derive useful information therefrom. The novel sections are especially intended for presenting seismic data in a form more suitable for analysis by an interpreter of such data.
(b) Prior Art
The general method of geophysical exploration utilizing seismic waves in the earth is well known. Briefly stated, this method comprises the steps of initiating a seismic impulse at or near the surface of the earth, and recording signals subsequently detected by geophones as a result of the earth's movements at one or more points suitably spaced from the point of origin of the seismic impulse. The original impulse will set up elastic waves that are transmitted through the earth. The signal recordation must permit measurement of the time elapsing between the start of the impulse and the generation of signals as a result of the subsequent earth movements. Any discontinuity or variation of structure within the earth will reflect and/or refract a portion of the energy in the waves, so that a recording of the signals from the geophones will comprise a number of arriving waves, each derived from the original impulse, and each differing from the others in time of arrival, magnitude, and wave shape, or in all three.
The usual type of seismogram produced by the technique described above, comprises a plurality of `line` traces, each trace consisting of a series of wavelets which vary in amplitude along a time axis from a common reference point which is the instant at which the impulse was initiated. Each reflected wavelet, in turn, consists of a positive and a negative lobe. Positive lobes are all portrayed on one side of the time or "null" axis, and negative lobes on the other side thereof. The points separating the positive and negative lobes are known as "null" points which lie on the null axis. Such line traces are recorded side-by-side, so that events produced thereon by reflections from a given discontinuity may be identified by the manner in which they align across the record. Manifestly, a multiplicity of line traces are required in order to distinguish between undesired noise occurrences and desired reflection events. Noise tends to produce random or discontinuous signals, whereas reflections from underlying strata tend to align on the seismogram as continuous signals.
Various seismic prospecting methods and instruments are employed to gather seismic data. Generally, the data from the seismic observations are recorded on a suitable recording medium which may be paper, film, magnetic tape, etc. Most observations, at the present time, are recorded in a digital format.
Each of these recording media is reproducible and will provide a seismogram, a record, or a section (these terms are used interchangeably herein) which are prepared and displayed as two-dimensional data graphs. The graph is a plot of a series of traces as a function of X and Y coordinates. The X or horizontal coordinate represents horizontal distance over the earth surface. The Y or vertical coordinate represents, alternatively, a time scale or a depth scale. The depth scale is calculated by determining approximate velocities which are converted to the depth scale by applying the simple formula: time multiplied by velocity equals depth.
A seismic section is commonly depicted as variations of black and white or, on occasion, as colored areas in vertical bands or traces. The section may consist of a few or several hundred such vertical traces. Each trace may represent seismic data from only one or from a plurality of seismic observations. The customary modes of display are known as variable area, variable density, or line.
Computer technology now permits a vast amount of seismic data to be manipulated expeditiously, and thus it is feasible to make various types of displays from the same raw data. The computer operations include: modifying the wavelet shapes; emphasizing certain portions of the seismic trace; attempting to suppress unwanted signals (noise); the ability to add the results of several trace recordings at one location, when the individual trace records are obtained from several different seismic impulse points (common depth point recordings); the ability to extract an approximation of the relative true amplitude events on the trace ("bright spot" technique); etc.
Inasmuch as the information of primary interest on a seismic section is concerned with the reflected events which are portrayed thereon, it is the general practice in the seismic art to use the records to determine the structural configuration (stratigraphy) and, if possible, the lithology of the sedimentary layers which lie below the earth's surface. Since a single 24-trace seismogram, a norm, may exhibit, more or less, 3000 separate and distinct events, it will be appreciated that a person interpreting seismograms is faced with the serious problem of selecting and evaluating significant information and discarding that which is insignificant. This procedure is extremely time consuming, expensive and tedious. The accuracy and interpretability of the information ultimately obtained is, to a great extent, controlled by the interpreter's knowledge, his experience, the quality of the raw data, the manner in which it has been processed, and by the presentation of the data in an understandable and interpretable form. It will also be appreciated that if the latter condition is not fulfilled, a full utilization and understanding of the data may not be achieved.
Primarily, a seismic section is used for the mapping of the subsurface structure. This involves identifying the same reflecting geologic horizon or horizons on one or on a series of sections (usually in the same general area and usually interconnecting), measuring the times from the earth's surface to this horizon, placing the times in their proper position on the map, and contouring the map. However, this seemingly simple procedure can become difficult because of lithologic and geometrical changes in the subsurface strata. Reflection continuity may be interrupted by faults, may terminate, may be effaced by noise, etc. Further, contiguous reflection signals from the same horizon may show differences in their amplitudes, frequencies, and shapes. Thus, in attempting to follow the same horizon, an interpreter frequently resorts to correlation. This is the process of comparing a series of wavelets on one trace with those on a non-adjacent trace, so as to be able to cross areas of discontinuity while still following the same reflection.